The invention relates to insulative and abradable ceramic coatings, and more particularly to ceramic turbine shroud coatings, and more particularly to a segmented ceramic coated turbine shroud and a method of making by plasma spraying or other line of sight deposition processes to form shadow gaps that result in a segmented morphology.
Those skilled in the art know that the efficiency loss of a high pressure turbine increases rapidly as the blade tip-to-shroud clearance is increased, either as a result of blade tip wear resulting from contact with the turbine shroud or by design to avoid blade tip wear and abrading of the shroud. Any high pressure air that passes between the turbine blade tips and the turbine shroud without doing any work to return the turbine obviously represents a system loss. If an insulative shroud technology could be provided which allows blade tip clearances to be small over the life of the turbine, there would be an increase in overall turbine performance, including higher power output at a lower operating temperatures, better utilization of fuel, longer operating life, and reduced shroud cooling requirements.
To this end, efforts have been made in the gas turbine industry to develop abradable turbine shrouds to reduce clearance and associated leakage losses between the blade tips and the turbine shroud. Attempts by the industry to produce abradable ceramic shroud coatings have generally involved bonding a layer of yttria stabilized zirconia (YSZ) to a superalloy shroud substrate using various techniques. One approach is to braze a superalloy metallic honeycomb to the superalloy metallic shroud. The "pore spaces" in the superalloy honeycomb are filled with zirconia containing filler particles to control porosity. These techniques have exhibited certain problems. The zirconia sometimes falls out of the superalloy honeycomb structure, severely decreasing the sealing effectiveness and the insulative characteristics of the ceramic coating. Another approach that has been used to provide an abradable ceramic turbine shroud liner or coating involves use of a complex system typically including three to five ceramic and cermet layers on a metal layer bonded to the superalloy shroud substrate. A major problem with this approach, which utilizes a gradual transition in thermal expansion coefficients from that of the metal to that of the outer zirconia layer, is that oxidation of the metallic components of the cermet results in severe volumetric expansion and destruction of the smooth gradient in the thermal expansion coefficients of the layers. The result is spalling of the zirconia, shroud distortion, variation in the blade tip-to-shroud clearance, loss of performance, and expensive repairs. Yet another approach that has been used is essentially a combination of the two mentioned above, wherein an array of pegs of the superalloy shroud substrate protrude inwardly from areas that are filled with a YSZ/NiCrAlY graded system. This system has experienced problems with oxidation of the NiCrAlY within the ceramic and de-lamination of ceramic from the substrate, causing spalling of the YSZ. Another problem is that if the superalloy pegs are rubbed by the blades, blade tip wear is high, causing rapid loss of performance and necessitating replacement of the shroud and blades.
Another reason that ceramic turbine shroud liners have been of interest is the inherent low thermal conductivity of ceramic materials. The insulative properties allow increased turbine operating temperatures and reduced shroud cooling requirements.
Thus, there remains an unmet need for an improved, highly reliable, abradable ceramic turbine shroud liner or coating that avoids massive spalling of ceramic due to thermal strain, avoids weaknesses due to oxidation of metallic constituents in the shroud, and minimizes rubbing of turbine tip material onto the ceramic shroud liner.